Refactor conversation pkg done

internal/conversation/conversation.go

@@ -190,9 +190,9 @@ type AssistantUpdate struct {
 func (AssistantUpdate) Type() EventType { return EventAssistantUpdate }
 
 // Done reports that the Inference the subscriber joined has returned.
-// Err is nil on success.
+// Error is empty on success.
 type Done struct {
-	Err error `json:"err"`
+	Error string `json:"error,omitempty"`
 }
 
 func (Done) Type() EventType { return EventDone }
@@ -297,7 +297,11 @@ func (inf *Inference) follow(
 			}
 		}
 		if done {
-			return send(ctx, ch, Done{Err: err})
+			var errStr string
+			if err != nil {
+				errStr = err.Error()
+			}
+			return send(ctx, ch, Done{Error: errStr})
 		}
 		// Wait for the call to close wait (when it appends new
 		// events or finishes), or for the subscriber to cancel ctx.